Where do mezcal flavors come from? In this ongoing series, Omar Muñoz explains the science behind the unique flavor profiles of your favorite mezcals.
In traditional mezcal production across Mexico, various types of containers are used to hold cooked agave for fermentation. The oldest documented containers are stone cavities, in a soil known as “tepetate,” located near rivers in Colima and Jalisco (CICY, 2012). These rock-carved cavities can also be found in Santiago Matatlán, Oaxaca, at a site called Los Pocitos.
In the villages near the Colima volcano, these pits are still in use. Throughout Mexico, fermentation may occur in buried and above-ground masonry tanks, cowhides, plastic containers, stainless steel containers, buried and above-ground clay pots, and especially wooden vats (mainly pine), or tinas.
Let’s briefly discuss alcoholic fermentation.
Alcoholic fermentation consists of four main stages:
First, during the latency phase, dormant yeasts adapt to their environment, consume oxygen, and produce the enzymes necessary to initiate fermentation.
Next, in the exponential phase, yeasts multiply rapidly, transforming sugars into alcohol (ethanol) and carbon dioxide, producing most of the alcohol and aromatic compounds.
In the stationary phase, the yeast population stabilizes due to nutrient and oxygen depletion, producing less alcohol but defining the final aromatic profiles.
Finally, in the decline phase, yeasts begin to die due to the lack of nutrients and the increasing alcohol concentration, releasing compounds that may influence the flavor of mezcal.
How do fermentation containers influence alcoholic fermentation and the flavors of mezcal?
Of the 97 compounds identified in different mezcals, 61 originate from fermentation (Barrón, 2012), meaning that 63% of mezcal’s flavors are formed during fermentation. This makes fermentation the most critical stage of the process, suggesting that the container in which it takes place is of utmost importance.
When considering how a fermentation vat might influence the flavor of mezcal, container porosity is one of the first ideas that comes to mind.
What does porosity have to do with the flavors of mezcal?
Porosity is the measure of empty spaces or pores within a material. It is expressed as a percentage and describes what fraction of the material’s total volume is composed of these pores. Simply put, it indicates how much “airiness” or void space exists within a solid.
Porosity is important in various fields, such as geology (to measure a rock’s capacity to retain fluids like water or oil), material science (to evaluate a material’s strength or permeability), and the manufacturing of products like filters or membranes, where pore size determines the flow of substances through the material.
Here is a table summarizing the main materials used for fermentation containers, their porosity, and pore sizes, which can later be correlated with the yeast sizes indicated in the last table.
| Material | Porosity (%) | Pore size (µm) |
| Plastic Containers | 0 | 0 |
| Stainless Steel Containers | 0 | 0 |
| Cowhides | 0-5 | 1-10 |
| Above-Ground Clay Pots | 10-20 | 1-30 |
| Buried Clay Pots | 15-25 | 1-50 |
| Tepetate (Volcanic Rock) | 5-15 | 10-100 |
| Wooden Vats (Pine) | 40-70 | 10-200 |
| Above-Ground Masonry Tanks | 5-20 | 30-300 |
| Buried Masonry Tanks | 10-25 | 50-500 |
Tepetate is a compact volcanic soil common in high-altitude areas of Mexico, characterized by high porosity due to natural fissures. Due to environmental humidity, buried masonry tanks exhibit greater porosity compared to the above-ground versions. Cowhides have low porosity, while plastic and stainless steel containers are impermeable and non-porous. Clay pots may exhibit varying degrees of porosity depending on their manufacturing process, allowing them to retain some moisture. Lastly, wooden vats, particularly those made of pine, have a highly porous structure that facilitates the absorption of liquids and gases.
How long do inactive yeasts remain viable during agave fermentation?
By the time mezcaleros begin distillation, fermentation is generally in its decline phase, leaving few active yeasts in the container. However, dormant yeasts remain present in the environment. When yeasts do not encounter favorable fermentation conditions, they slow their metabolism, entering a period of inactivity. During this phase, they may occupy the pores of fermentation containers to shelter themselves, awaiting suitable conditions to reactivate.
The duration for which an inactive yeast can remain viable depends on several key factors. Temperature plays a critical role: low, constant temperatures, such as those in refrigeration (4–10 °C), significantly extend their viability, while high or fluctuating temperatures accelerate their degradation. Relative humidity also influences viability, as dry environments better preserve yeasts, while high humidity can cause partial rehydration, damaging the cells. The yeast’s initial physiological state before inactivation, along with the availability of nutrients or protective substances in its environment, further determines how long it can remain viable. Finally, the yeast species is important; for instance, Saccharomyces cerevisiae is more resilient compared to species like Hanseniaspora spp.
An optimal combination of these factors can extend the viability of inactive yeasts for several months or even years. However, in traditional palenques, none of these factors are controlled.
The time that yeasts remain viable in their inactive state is crucial for determining their influence on mezcal flavors, as it directly affects the time it takes for yeasts to inoculate the agave must.
What is the latency phase in agave fermentation?
As previously mentioned, the latency phase is the time it takes for yeasts to adapt to their environment, and it depends on various factors. Environmental conditions are critical for rapid activation: pH, sugar concentration, temperature, and nutrient availability. The yeast’s physiological state also plays a role; young, well-nourished yeasts have a shorter latency period compared to weakened ones. This phase generally begins once the cooked agave is crushed and placed in the fermentation vats.
The type of yeast is also relevant, as cultivated yeasts tend to activate more quickly than wild strains. Initial oxygen availability is important for yeast reproduction before the anaerobic fermentation stage, and temperature also plays a role: lower temperatures extend latency, while higher temperatures shorten it, as long as they remain within the yeast’s tolerable range.
For example, let’s take a palenque that produces mezcal only a couple of times during the dry season. In such palenques, vats are often allowed to dry out, and temperature fluctuations are significant. Under these conditions, it is likely that the yeasts in the vats will no longer be viable after about three months. In this case, the material of the vat becomes irrelevant. Therefore, the frequency of mezcal production in the palenque is a more critical factor for yeast survival in the pores of the containers than the material itself.
However, in palenques with consistent activity, porous materials will harbor inactive yeasts in good condition, which will accelerate the latency phase.
How the exponential phase of agave fermentation influences the flavor of mezcal
During the exponential phase of fermentation, active yeasts begin transforming sugars into ethanol and secondary compounds, producing the majority of the characteristic aromas and flavors of mezcal. Suboptimal conditions, such as a lack of oxygen or nutrients, can disrupt the balance between aerobic and anaerobic fermentation, impacting the production of esters, higher alcohols, and other compounds that define the sensory profile.
The efficiency and speed of this phase are crucial for developing fruity, floral, and complex notes in mezcal. Conversely, prolonged periods or stress on the yeasts can result in undesirable flavors, such as sulfurous or bitter notes. The porosity of the fermentation container material affects the availability of oxygen and nutrients, which are essential for optimal yeast reproduction.
However, in traditional mezcal production, where fermentation is naturally inoculated by the environment, the exposed surface area of the container is more significant than the material’s porosity in influencing fermentation dynamics. This emphasizes the role of the open-air environment and microbial ecosystem in shaping the mezcal’s unique sensory characteristics.
Can the size of the fermentation vat influence the flavor of mezcal?
With open fermentation, the container’s size significantly impacts the mezcal flavor by influencing microbial metabolism, the dynamics of volatile compounds, and environmental interactions.
Large containers have a lower surface-to-volume ratio, which limits oxygen exchange and promotes more intense and complex aromatic profiles. However, they are more prone to thermal and nutrient stratification. This means that in large containers, the temperature is not kept uniform, and instead, overlapping temperature layers are formed. This is because warmer air is less dense and tends to rise, accumulating in the higher parts. And so does the availability of nutrients in the fermentation vat. On the other hand, smaller containers, with a larger surface area exposed to air, facilitate aerobic microorganism activity but may also lead to the loss of volatile compounds.
The microbiota in large containers tends to be more diverse, enriching the organoleptic profile of the mezcal, while in smaller containers, it stabilizes more quickly, resulting in more uniform fermentations. Additionally, the temperature tends to remain more stable in large containers, supporting uniform yeast metabolism, whereas in smaller containers, it can rise more rapidly, potentially causing yeast stress and affecting fermentation outcomes.
The “formulation” of mezcal
An often-overlooked aspect of mezcal production involves the yeast’s resistance to what is known as “shear stress.” Shear stress can be defined as the yeast’s ability to withstand “impacts” without membrane damage or cell death.
In most traditional mezcal distilleries, milled and cooked agave fibers are left to sit in the vat while producers wait for them to “heat up” before adding water. When the mezcaleros feel that the fibers are already hot, they add water to the fermentation vessel and then stir the mixture. A common name for this step is “formulation.”
Formulation is crucial because, depending on how vigorously the solution is agitated (which is usually done with wooden paddles), yeasts that are less resistant to these physical impacts are killed. As a result, the sensory profile of the mezcal is heavily influenced at this stage.
Furthermore, in many cases, maestros mezcaleros immerse their hands into the mixture, literally leaving their personal touch on the process. This emphasizes how traditional techniques that are unique to each region continue to play a significant role in defining the distinct profiles of every mezcal!
Below is a table of different yeasts reported in natural fermentations, listed according to their resistance to shear stress and other important variables.
Do the organic and mineral elements of leather, masonry, wood, and clay fermentation vats influence the flavor of mezcal?
Let’s begin with cowhide fermentation containers, as their impact on the mezcal profile is particularly noticeable. This is due to the natural presence of lactic acid bacteria (LAB), such as Lactobacillus plantarum, Weissella cibaria, and Leuconostoc mesenteroides (Espinoza-Martínez, 2020; Alderete-Tapia, 2018), as well as acetic acid bacteria (AAB) of the Acetobacter genus (Alcázar-Valle, 2015). These microorganisms play a significant role in producing acetic acid (vinegar) and lactic acid, which contribute to what most people perceive as lactic and wet leather aromas and flavors, depending on their concentration.
While these bacteria can be present in any fermentation setting, their influence is most pronounced when cowhides are used as fermentation containers. The unique microbial ecosystem on the hides amplifies these sensory characteristics, leaving a distinct mark on the final profile of the mezcal.
In the case of wooden vats, particularly new ones, they may release tannins (especially if made from oak) and lignins. Tannins are natural compounds found in plants, fruits, bark, and leaves. They are responsible for the bitter or astringent taste you feel when consuming foods like red wine, tea, or unripe fruits like persimmons. Tannins are high molecular weight with low volatility, making it unlikely for them to be carried over during distillation. For this reason, tannins are not found in the final distilled mezcal.
Lignins are organic compounds found in the woody tissues of plants. Although lignins are also non-volatile, they can break down during fermentation into smaller, volatile compounds such as vanillin or guaiacol (Silvia Pérez-Magariño, 2020). These compounds are carried over during distillation, contributing aromatic notes to the mezcal, such as vanilla or smoky tones. However, as the wooden vats are repeatedly used, these properties gradually diminish, reducing their direct impact on the mezcal’s flavor over time.
The use of clay containers, cavities, and masonry in fermentation can release minerals such as calcium, magnesium, or iron, which influence the pH and serve as micronutrients that modify the metabolic behavior and enzymatic action of the yeasts. Additionally, their porosity (depending on their manufacturing) allows for the adsorption and gradual release of volatile compounds.
The micronutrients provided by these materials influence the diversity of compounds produced by yeast; however, these micronutrients can also be obtained from the water used during fermentation. This means that, in many cases, the water used for fermentation is more important than the material of the fermentation vessel.
Clay pots are often small, and the formulation is done manually, so the size of the pot and the survival of more yeasts due to the lower shear stress they experience may be the factors that most impact the sensory characteristics of these mezcals.
In contrast, stainless steel and plastic containers provide a more controlled and sterile environment, reducing the influence of residual microorganisms and allowing for more consistent fermentation, though generally with less complexity in the flavor profile.
What can we conclude about how much fermentation vats influence the flavor of mezcal?
At the palenque of brothers Fernando and Gustavo Ortega in Santa Catarina Albarradas, Oaxaca, fermentation is carried out in leather, above-ground masonry, and plastic containers. According to them, both leather and plastic containers “heat up” in similar timeframes, which is likely due to their low porosity and comparable size. However, when it comes to flavor, a significant change is only noticeable when fermenting in leather, while fermentation in masonry and plastic does not yield notable differences. This observation aligns with the concepts discussed in this article.
Impartially speaking, the wild yeasts present in each location and the traditional techniques of each village do more to shape the diverse sensory profiles of traditional mezcals than the material of the fermentation containers. To quantitatively determine the exact impact of different containers on flavor, extensive additional experiments would be necessary.
However, mezcal is more than just a beverage; it is a living testament to the interplay of nature, tradition, and technique. Each fermentation container, with its unique material, size, and porosity, not only influences how the yeasts and bacteria interact to define the aromatic profiles but also captures the essence of the region and the traditions passed down through generations. The true magic of mezcal lies in how these factors combine to tell stories through flavors and aromas that cannot be replicated in an industrial context.
Thus, the influence of the container transcends mere functionality; it becomes a tangible link to the past and a reminder of human ingenuity in harmony with nature. When savoring mezcal, we are not merely enjoying an artisanal drink, we are tasting the soul of a community, its landscapes, its traditions, and its cultural identity. It is this delicate balance between science and tradition that makes mezcal a heritage worthy of admiration and protection.
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